1. Field of the Invention
The present invention relates to a resist suitable for forming patterns during the manufacture of semiconductor devices, particularly the manufacture of large-scale integrated (LSI) circuits.
2. Description of the Related Art
Photolithography is used in some steps of manufacturing a semiconductor device such as an LSI circuit. This technology is to etch substrate (e.g., a silicon single-crystal wafer), thereby to form fine lines and small windows. More specifically, a photoresist film is formed on the substrate by means of spin coating, and is then exposed to light applied through a pattern mask. The film is developed and rinsed, and is thereby made into a resist pattern. Then, the substrate is etched by using the resist pattern, thereby forming fine lines and small windows in the surface of the substrate.
It is increasingly desired that LSI circuits have higher integration densities. Here arises a demand for technology which can form lines and windows in a substrate, which are finer and smaller than those made by the existing photolithography. To meet this demand, use is made of a light source which can apply a light beam having as short a wavelength as is possible to a photoresist film. More precisely, a deep UV source, such as an KrF excimer laser or an ArF excimer layer, is employed in photolithography.
The conventional resist absorbs light having a short wavelength, so much that the sufficient light fails to reach that surface of the resist film which faces away from the light-receiving surface. After the resist film is developed and rinsed, it will have openings inevitably having a cross section shaped like an inverted triangle. Consequently, when the substrate on which the resist film is formed is etched, using the resist film as etching mask, an original LSI is not transferred to the substrate, with sufficient accuracy.
To solve the problem with the conventional resist, it has been proposed that so-called chemically amplified resist be utilized. A chemically amplified resist contains a compound which generates strong acid when exposed to light (hereinafter referred to, when necessary, as "acid generator"), and also a compound in which hydrophobic groups are decomposed by acid and which is thereby changed into a hydrophilic substance various resists of this type are known. To be more specific, U.S. Pat. No. 4,491,628 (1985) to H. Ito, C. G. Wilson, and J. M. J. Frechet discloses a positive resist containing a polymer wherein the hydroxyl group of poly(p-hydroxystyrene) is blocked by butoxycarbonyl group, and onium salt which generates an acid when exposed to light. Also, M. J. O'Brien, J. V. Crivello, SPIE, Vol. 920,Advances in Resist Technology and Processing, p. 42, (1988) discloses a positive resist containing m-cresol novolac resin, naphthalene-2-carboxylic acid-tert-butylester, and triphenylsulfonium salt. Further, H. Ito, SPIE, Vol. 920, Advances in Resist Technology and Processing, p. 33, (1988) discloses a positive resist containing onium salt and 2,2-bis(4-tert-butoxycarbonyloxyphenyl) propane or polyphthalaldehyde.
The acid generator acts as a catalyst, efficiently reacting with other substances. Hence, when light is applied to the resist film, chemical reaction thoroughly takes place even in that surface of the film which faces away from the light-receiving surface and to which the light can hardly reach. As a result, the resist film can be made, after being developed and rinsed, into a resist pattern having openings which has steep sides.
Each of the resists specified above contains onium salt used as an acid generator. Onium salt includes arsenic or antimony which are harmful to human. In view of this, it would be questionable to put onium salt to industrial use. In case arsenic or antimony remains on the substrate (e.g., a wafer) after the resist pattern has been removed, it will probably impair the characteristics of the electronic circuit to be formed on the substrate. Resist containing onium salt has another disadvantage. In the surface of a film of this resist, there will probably be formed a layer which is hard to dissolve, inevitably making scars in the upper surface of the resultant resist pattern. Such scars are detrimental to the subsequent etching of the substrate.